Homeostasis maintains a relatively constant internal environment through control systems. These control systems operate by feedback. They compare the current state of the organism with a desired state. If a stimulus disrupts a controlled condition, control loops restore the disturbance to normal values. Negative feedback systems keep controlled conditions stable around a setpoint.
A good example of negative feedback is the control of body temperature. Body temperature is very closely regulated by negative feedback. The temperature control center is in the hypothalamus, which monitors both blood and skin temperature. If the blood is too warm, the hypothalamus responds by promoting heat loss to reduce the temperature. The primary effects are due to evaporation of sweat and vasodilation of blood vessels in the skin. If the blood temperature drops slightly, the response is heat conservation. Blood is diverted to the core, we move and put on clothing to keep warm, and may start shivering.
Pyrogens produced during a fever reset the thermostat to a higher set-point temperature. This may cause us to feel cold, even when our body temperature is elevated. So, we may be running a fever and still feel chills and shiver.
Positive feedback responds to a stimulus by making the change larger and larger. Positive feedback needs an end-point, a means of escape from the cycle. An example of positive feedback is the uterine contractions that lead to birth. As the cervix is stretched by the baby’s head, stretch receptors respond by sending a signal to the hypothalamus, at the base of the brain. The hypothalamus releases oxytocin into the posterior pituitary gland, where it is taken up into the blood stream and travels to the uterus. The uterus responds by contracting.
The contractions cause the baby’s head to push more on the cervix, amplifying the cycle of oxytocin release and further pushing against the cervix. The escape from this positive feedback cycle is birth.
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